WCSP eliminates conventional packaging steps such as die bonding, wire bonding, and die level flip chip attach processes to a package substrate by using the IC die itself as the WCSP substrate. Use of the die itself as the WCSP substrate significantly reduces the footprint to the IC die itself as compared to the same IC die attached to a larger footprint package substrate.
WCSP can be embodied as direct-bump WCSP or redirect layer (RDL) WCSP which unlike direct-bump WCSP adds an RDL that functions as a rewiring layer that enables repositioning external terminals at desired positions. In a typical RDL WCSP production flow, after completion of conventional back end of the line (BEOL) wafer fab processing, the IC die generally includes die pads (also known as bond pads or die bond pads) and a passivation layer over the BEOL metal stack except over the die pads. A first WCSP dielectric (e.g., a polyimide) is deposited. Lithography/etch forms first vias in the first WCSP dielectric over the die pads, followed by deposition and patterning of an RDL metal including a plurality of RDL traces which contact the die pads and extends laterally therefrom.
A second WCSP dielectric (e.g., a polyimide) is then deposited and second vias formed that reach the RDL in RDL capture pad positions that are lateral to the position of the die pads. Under bump metallization (UBM) pads commonly referred to as “ball pads” or “bump pads” are formed over the second vias and are coupled to and generally enclosed by RDL capture pads, followed by forming metal (e.g., solder) balls, pillars or other bonding connectors on the UBM pads. The area of the RDL capture pads is generally larger than the area of the UBM pad thereon to absorb stresses and thus improve reliability. The WCSP wafer is then singulated to form a plurality of singulated WCSP die, commonly for use on boards for portable devices where the board area is precious.
Each bonding connector, such as a solder ball, provides a repositioned I/O connection for the WCSP die. Thus, the “ball count” commonly refers to the total number of I/O connections on the WCSP die. The I/O connections are electrically tied together into a plurality of different “nets”, such as at least one VDD net, one or more signal nets, and a VSS (ground) net. The balls associated with the respective nets can be tied by one of the metal layers on the die, the RDL, or a combination of both of these arrangements.
The number of balls or other bonding connectors that can be fit on the WCSP die is determined by clearance design rules that define the minimum pitch between the respective balls or other bonding connectors, the “ball to pad” design rule that defines the minimum spacing between bonding connectors and die pads associated with different nets, and the requirement that the UBM pad under the bonding connectors be completely enclosed by the RDL. The first and second of these design rules are intended to prevent leakage or shorts between different nets, with the third requirement for improved reliability.
One solution for increasing the number of balls or other bonding connectors on the WCSP die involves positioning some of the die pads between rows or columns of the bonding connectors. This solution can alleviate the problem of die size growth, but requires a die pad or I/O layout change which necessitates changing one or more die masks.